The present invention relates to constructing roads. More specifically, the present invention relates to a method of selecting an asphalt binder for use in constructing a high modulus layer of a roadway. Still further, the present invention involves formulating a binder for a roadway high modulus layer by mixing a stiff asphalt binder and a soft asphalt binder together.
The base layers of a roadway must be stiff enough to prevent high levels of bending or flexing under heavy traffic loads. They also must be able to properly distribute and spread the loads imparted to the substructure of a pavement system. One commonly employed method of controlling the amount of bending in the base layers and ensuring their capacity to distribute loads is to increase the overall thickness of the pavement structure. One disadvantage with such a method is that it can be very expensive. If thinner pavement cross-sections are made, costs can be reduced, but this typically will increase the flexing of the pavement structure and lead to rapid fatigue damage of the roadway when conventional materials are used.
Another way of creating a pavement layer with good properties is to select a desirable binder. Binders have been chosen that have a particular penetration value at a particular temperature, such as between 0 and 20 dmm at 25° C. However, relying solely on the penetration value of a binder does not adequately predict the performance of the pavement layer being created. More specifically, the penetration value by itself does not adequately capture both the stiffness requirements needed for maximizing bending stiffness and the minimum low temperature flexibility requirements needed so that thermal cracking does not become an overriding consideration.
Another methodology for selecting a binder is to measure its complex shear modulus at temperatures of about 50° C. and higher. The current industry-practice protocol for evaluating the performance of asphalt binders for resisting permanent deformation is the Strategic Highway Research Program (SHRP) Performance Grading (PG) Methodology (AASHTO M320 entitled Performance-Graded Asphalt Binder). In the SHRP PG protocol, the ability of a binder to resist permanent deformation is determined by measuring the temperature at which the unaged asphalt binder's complex shear modulus divided by the sine of the phase angle (G*/sin delta) is at least 1.0 kPa when measured at a frequency of 10 rad/sec. The phase angle and G* are measured beginning at approximately 52° C. and increasing in 6° C. increments therefrom. For example, an asphalt binder's phase angle and G* might be measured at 52° C., 58° C., and 64° C. If G*/sin delta for this binder is 2.75 kPa, 1.3 kPa, and 0.77 kPa at 52°C., 58° C., and 64° C., respectively, then the binder is classified as a PG-58 binder. This grading implies that the binder possesses a minimum required G*/sin delta to resist rutting at a pavement temperature of up to 58° C. One disadvantage with such a method is that it is not adequate for evaluating a high modulus layer binder because it does not sufficiently evaluate the contribution of the binder to help prevent excessive bending or flexing of the pavement at temperature ranges of interest.
Binders also have been selected using their ring and ball softening points. However, such data alone does not sufficiently predict whether the binder will adequately contribute to distributing the loads applied by heavy vehicles. In summary, the binder specification and formulation protocols of the prior art do not consistently provide pavement layers that meet desired pavement design requirements.
In order to overcome these disadvantages, a method of selecting an asphalt binder for a high modulus pavement layer that better predicts the ability of the binder to help minimize pavement bending or flexing is needed. A method of selecting a high modulus layer binder that better predicts the ability of the binder to distribute loads imparted to the substructure of the pavement system also is needed. Still further, a reproducible, reliable, and efficient method of formulating and testing high modulus layer asphalt binders is desired.